Method of hot pressing titanium diboride utilizing a growing sintered zone



Aprll 7, 1970 R. H. BIDDULPH 3,505,438

METHOD OF HOT PRESSING TITANIUM DIBORIDE UTILIZING A GROWING SINTERED ZONE Filed June so, 1967 0 O MAM/.5 MAM/.5 MAM/s INVENTOR. Ea PEA/4P0 mMProA/ a/owuw United States Patent O 3,505,438 METHOD OF HOT PRESSING TITANIUM DIBORIDE UTILIZING A GROWING SINTERED ZONE Richard Hampton Biddulph, Worcester Park, England, as-

signor to United States Borax & Chemical Corporation, Los Angeles, Calif. Continuation-impart of application Ser. No. 441,441, Mar. 22, 1965. This application June 30, 1967, Ser. No. 650,278

Int. Cl. C04b 33/32, 35/64 US. Cl. 264-66 3 Claims ABSTRACT OF THE DISCLOSURE An improved method of hot pressing titanium diboride wherein the improvement comprises heating the entire body under pressure to a temperature below that required to bond and densify the paticulate material, then further heating only an intermediate zone of the body to a temperature about 500 C. higher than that of the end portions and sufficient to bond and densify the intermediate zone, and finally gradually expanding this intermediate zone to the ends by increasing the temperature of the end portions.

This application is a continuation-in-part of my copending application Ser. No. 441,441, filed Mar. 22, 1965, now Patent No. 3,336,431.

This invention relates to electric furnaces of the resistance type and process for hot-pressing refractory materials.

It has been proposed in copending application Ser. No. 193,793 filed May 10, 1962, by A. A. R. Wood and B. P. Long, now Patent No. 3,246,956, to employ as the heating element of an electric-resistance furnace a cylindrical graphite tube having two diametrically opposed longitudinal slits extending from one end almost to the other. Current is led to the element by two metal contact blocks, one on each limb of the slotted end of the element. While the temperature profile within such a furnace can be varied to some extent by altering the applied or by shaping the element, the possible variation is very limited and difiicult to control.

I have now devised a new electric furnace which enables considerable variation of the temperature profile therein conveniently to be obtained and which, in certain constructions, is especially suitable for use in the hotpressing of refractory particulate materials.

The electric furnace of this invention comprises an electric-resistance heating element of generally tubular shape, the wall of the element being interrupted at one or both ends by at least two longitudinal gaps (hereinafter referred to as slots) extending part way toward the unslotted end of the element or part way toward the inner end of the slots running from the other end of the element, each limb of the slotted end or ends of the element and any unslotted end being provided with an electtrical contact. While the resistance element may be slotted at only one end, preferably both ends are slotted. Elements slotted at both ends not only enable a greater variation in temperature profile to be obtained than when only one end is slotted, but also are particularly suitable for use in the method of hot-pressing refractory powders described in detail below.

The temperature profile within the element can be conveniently varied by superimposing local heating currents on the main heating current. The electrical resistance of the element between contacts on different limbs of the same end of the element is desirably lower than the resistance between contacts at opposite ends of the element. Preferably one or more sections of the wall of the 3,505,438 Patented Apr. 7, 1970 element are thinned or completely cut away. In the case of elements slotted at both ends, an intermediate section of the element between the inner ends of the slots is desirably pierced by a continuous spiral cut or other gap around the periphery of the element. Instead of the spiral cut, a number of longitudinal or transverse slots may be cut in the intermediate section.

The element is desirably made of graphite, although other conducting materials capable of withstanding the intended operating temperature of the furnace may be used. Generally, the element will be circular in crosssection, although this is not essential. The slots at one or both ends of the element are preferably symmetrical but they may be unsymmetrically arranged and are conveniently parallel to the axis of the element.

Aluminum, copper or other metal blocks are suitable for use as the contacts. Desirably they are cooled, as by cooling jackets through which water or another fluid is passed.

Furnaces according to the invention in which both ends of the element are slotted and the resistance from end to end is higher than the resistance across each slotted end are especially suitable for use in the hot-pressing of powders of refractory materials, such as the refractory carbides, silicides, nitrides and borides, for example titanium diboride, boron carbide and boron nitride.

It has been found, according to a further aspect of the invention, that it is advantageous when hot-pressing particulate refractory materials by means of relatively movable pressure surfaces acting upon the ends of a mass of the material which is laterally confined, as in a graphite or other heat resistant die, to apply heat at first so that only a zone of the mass between the ends is at a temperature at which bonding of individual particles occurs and then so that each longitudinal boundary of the zone moves gradually outwards to the end of the mass. The furnace herein described is particularly adapted to carrying out this process. This process enables compacts of excellent density to be obtained with relatively short pressing times.

The furnace is illustrated in the accompanying drawing in which FIG. 1 is a sectional side view of one form of furnace according to the invention and FIG. 2 is an overall view of the furnace and diagram of a circuit suitable for operating this furnace.

Referring to FIG. 1 of the drawing, the furnace comprises an outer graphite tube 1, a tubular graphite resistance heating element 2 and contacts 3a, 3b, 3c, and 3d in the form of metal blocks in electrical contact with the ends of the resistance element. At the ends of the element each limb is in contact with one of the contact blocks 30, 3b, 3c, and 3d. Water cooling rings 4a and 4b are provided for supporting the ends of the outer graphite tube. A tubular casing 5 encloses the outer graphite tube 1. The free space within the casing is packed with carbon black (not shown) for heat insulating purposes. The contact blocks (3a, 3b, 3c, and 3d) are separated from the casing by an insulating washer at each end of the furnace (6a and 6b).

In FIG. 2, the wall of the heating element 2 is interrupted at each end by two diametrically opposed longitudinal slots 7 extending part way towards the inner ends of the slots running from the distant end of the element. The intermediate section of the element inwards of the slots contains a peripheral continuous spiral cut 8.

The circuit shown in FIG. 2 includes three transformers T T and T with low-voltage secondary windings, used to supply power to the furnace. One end of the secondary winding to T is connected to contact blocks 3a and 3b via a center tap on the secondary winding of transformer T and the other end to the blocks 30 and 3d via a center tap on the secondary winding of transformer T The ends of the secondary winding of T are connected across the contact blocks 3a and 3b and the ends of the secondary winding of T are connected across the contact blocks 3c and 3d at the other end of the element. The ends of the primary windings of transformers T T and T are connected across the mains through variable transformers 9 to act as controllers. (Saturable reactors are also suitable for use as controllers.) The output voltage of T is several times the separate output voltages of T and T and the output voltages of T and T are substantially equal.

The intermediate section of the element containing the spiral cut 8 has a much higher resistance than either outer section. Consequently, if at first transformer T alone is used to supply power to the furnace, the temperatures attained in the center section are much higher than those attained in the outer sections. If then power is also supplied by transformers T and T the temperatures of outer sections rise and the boundaries of the high temperature zone in the furance each move gradually towards the ends. This mode of operation of the furnace is satisfactory for hot-pressing refractory particulate materials according to the procedure described above. Other variations of the temperatureprofile within the furnace may be achieved by appropriate switching-in, switching-out or alteration of the power input to the three sections of the furnace.

In a typical example of the process of this invention, approximately 30 pounds of titanium diboride powder of mean particle size microns were placed in a 60 inch die of 3 inches diameter. The material was added to the die gradually and carefully tapped down by hand. The die was surrounded by a furnace of this invention as shown in FIGS. 1 and 2 and a pressure of 500 p.s.i. applied. The material in the d e at this stage extended for a distance of 38 inches in the die. The whole of the material was raised to a temperature of 1500 C. and the applied pressure increased to 2250 p.s.i. The center section of 9 inches was raised to 2000" C; by means of increasing the current passing through the central section and the ends were maintained at 1500 C. by reducing the power input to the ends. After 2 hours, the outer sections were raised to 2000" C. by increasing the current to these parts and the current in the central section reduced so as to maintain the same temperature. The pressure was maintained and heating continued until no further movement of the rams took place. Upon cooling, the resultant bar (24 x 3 inches) was found to be of regular cross section over its length to within 1% and to have a density of 97% throughout.

A bar similarly pressed but using a furnace with no temperature profile control had a mean density of 93%, the ends being of lower density than the central section. In the middle, it had a diameter of only 95% of the ends.

Thus, by employing the process of the present invention it is possible to obtain a product of higher density and the finished product is of more uniform cross section.

Various changes and modifications of the invention can be made and, to the extent that such variations incorporate the spirit of this invention, they are intended to be included within the scope of the appended claims.

What is claimed is:

1. In the process for hot-pressing particulate titanium diboride which includes the steps: charging said particulate titanium diboride to a mold having means to apply pressure to the charged mass While laterally confined, and applying heat and pressure to said laterally confined mass sufiicient to bond and densify said mass, the improvement which comprises heating an intermediate zone of said mass between the ends at a temperature about 500 C. higher than the temperature of the end portions to effect bonding and densification of the intermediate zone and then expanding each longitudinal boundary of the bonding and densification zone gradually outward to the ends of the mass.

2. The process according to claim 1 in which said mass is heated initially at about 1500 C., said intermediate zone is then heated at a higher temperature of about 2000" C. and the higher temperature zone is gradually expanded outwardly to the ends of the mass.

3. The process according to claim 1 in which said pressure is about 2250 p.s.i,

References Cited UNITED STATES PATENTS 2,027,786 1/1936 RidgWay et al. 264--332 2,167,544 7/ 1939 De Bats et al 2*64-332 2,535,180 12/1950 Watson 264332 2,920,353 1/ 1960 Strating et a1. 264-327 3,003,885 10/1961 Mandorf 264-332 3,116,137 12/1963 Vasilos et al 2 64-632 3,143,413 8/1964 Krapf 10643 3,248,215 4/1966 Bonis et a1 264332 3,346,681 10/1967 White et a1. 2'64332 JULIUS FROME, Primary Examiner I. MILLER, Assistant Examiner US. Cl. X.R. 

